Recently, the substitution of glass with transparent materials which do not shatter, or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles such as trains, buses, taxis and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in fuel economy.
While transparent plastics provide a major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as LEXAN.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature and good dimensional stability. It is also self-extinguishing, and easily fabricated. Acrylics, such as polymethylmethacrylates, are also widely used glazing and lens materials.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. Misch, et al., U.S. Pat. No. 3,708,225; Clark, U.S. Pat. Nos. 3,986,997, 3,976,497 and 4,027,073; Armbruster, et al., U.S. Pat. No. 4,159,206; and Ubersax, U.S. Pat. No. 4,177,315, for example, describe such compositions. Improved such compositions are also described in commonly assigned copending U.S. application Ser. No. 964,910 filed Nov. 30, 1978, abandoned in favor of Ser. No. 415,844 filed Sept. 8, 1982, and Frye, U.S. Pat. No. 4,277,287.
It has been discovered that such polysilicic acid coatings, especially if acidic, fail to adhere to certain plastic substrates, such as polycarbonate, and, even if prepared on the basic side of neutrality, they may adhere initially, but they will peel after brief light aging. In copending application Ser. No. 91,716, filed Nov. 6, 1979, now U.S. Pat. No. 4,299,746, the addition of an ultraviolet (UV) light absorbing agent, such as 2,4-dihydroxybenzophenone, is suggested, but in some cases this may have a plasticizing effect and polycarbonate seems to have a tendency to reject the coating on severe exposure. Another approach is to use a primer coat which adheres to both the silicone resin coating and the polycarbonate substrate, and also serves as a binder for high levels of UV absorbers. The second coat is put on over the primer coat. See, for example, commonly assigned Humphrey, Jr., U.S. Pat. No. 4,188,451.
To avoid the need for a primer coat, commonly assigned copending U.S. application Ser. No. 34,164, filed Apr. 27, 1979, now U.S. Pat. No. 4,413,088 suggests replacing the usual solvents, e.g., isobutanol, with a more aggressive solvent, e.g., an ester, a ketone, a nitroparaffin, or the like. However, these are expensive and generally might require process modifications.
Other approaches to the elimination of primers from the application of silicone coatings to plastic are described in commonly assigned copending U.S. application Ser. Nos. 326,197, filed Dec. 1, 1981 and 154,624 filed May 30, 1980, now U.S. Pat. No. 4,373,061. The first-mentioned describes shock curing of silicone resin coatings at elevated temperatures to yield an adherent coating without the use of primers. The latter application discloses that certain UV screener-functionalized silicone resin coating compositions admit to primerless adhesion to plastic, although a lengthy aging period may be required before these compositions are ready to use.
The above-mentioned patents and applications are incorporated herein by reference.